Peak detector for detecting accurate pulse peaks produced by a magnetic head exhibiting residual magnetism

ABSTRACT

A peak detector for detecting accurate peaks of pulses recovered from a magnetic disk in the presence of residual magnetism by a magnetic head, but without the deleterious effects of such residual magnetism. The head supplies differential pulses as respective pulse trains to first and second peak detectors, respectively. The peaks detected in the first train of differential pulses are combined with the peaks detected in the second train, and an output signal produced as a function of the combined peaks is indicative of the pulse peaks. The respective peaks, which represent the positive and negative peaks of the pulses, are compared to a set of reference levels and, depending upon the relationship of those peaks with the reference levels, a gain error or an offset error is indicated. If the positive pulse peaks and negative pulse peaks exhibit substantially similar relationships with respect to the reference levels, a gain error is indicated. However, if the positive pulse peaks and negative pulse peaks exhibit dissimilar relationships with respect to the reference levels, an offset error is indicated.

This application is a continuation of application Ser. No. 07/769,405,filed Oct. 1, 1991 abandoned.

BACKGROUND OF THE INVENTION

This invention relates to pulse detection and, more particularly, to apulse detector which accurately detects pulses read from a magnetic diskby a magnetic head which may exhibit residual magnetism. The inventionalso relates to a discriminator which uses the detected pulses todiscriminate between offset errors and gain errors in the pulsedetecting channel.

In conventional hard disk drives, servo information typically isrecorded for the purpose of maintaining the head or heads of the drivein accurate registration with a track being scanned. In earlier harddisk drives comprised of multiple disks, it was conventional to dedicatethe entire surface of one disk to servo information; and a separateservo head was used to read that information from which positional, ortracking errors were detected. A closed loop feedback arrangementadjusted the servo head relative to the servo tracks being scannedthereby so as to correct for tracking errors. Since the servo head wasincluded in a stack of heads, tracking error correction of the servohead resulted in tracking error correction of all of the heads.

More recently, servo information has been disposed in limited portionsof each track on each disk surface; and the same head normally used toread or write useful data is used to read this servo information.Typically, a servo pattern is recorded in a sector header, with headersbeing disposed uniformly in each track. Although various types of servopatterns have been proposed, their primary objective is to produce asignal which represents the magnitude and direction of a tracking error.One type of servo pattern produces a pair of positive pulses ofintermediate amplitude followed by a single negative pulse of largemagnitude when scanned. If the head drifts to one side of the track, theamplitude of one of the positive pulses exceeds the amplitude of theother, and the amplitude of the negative pulse is reduced. Hence, thedirection of the tracking error is detected as a function of whichpositive pulse amplitude increased, and the amount of this error isdetected as a function of the difference between the peaks of thepositive pulses.

Another type of servo pattern is formed as two (or more) bursts ofmagnetic domains, both offset from the center of the track in oppositedirections. Because of this equal offset, when the head is centered onthe track, the pulses derived from one burst will be of equal amplitudeto the pulses derived from the other burst. If the head drifts to oneside of the track, the pulse amplitudes derived from one burst will begreater than the pulse amplitudes derived from the other. Thus, bydetermining which of the bursts results in greater pulse amplitudes, thedirection of the tracking error is detected. Similarly, the magnitude ofthis tracking error is sensed as a function of the difference betweenthose pulse amplitudes. Typically, amplitudes of the pulses derived fromthe bursts are sensed by peak detection. Since each magnetic domainresults in a pulse pair, one being positive-going and the other beingnegative-going, the pulses first are full-wave rectified and then thesefull-wave rectified pulses are detected.

In the foregoing peak detection arrangement, it is expected that thepositive excursions above a base line (or AC reference level) producedwhen a burst of magnetic domains is scanned is equal to the negativeexcursions below that base line. However, it has been found in practicethat the head which reads the servo pattern may exhibit residualmagnetism. As a result, the positive excursions above the base linediffer from the negative excursions below. This residual magnetism maybe thought of as a bias on one side of the head gap but not the other,and may be produced during a data write operation. Data normally iswritten on a track until a servo pattern is reached, whereafter the samehead is changed over to its read mode to sense the servo pattern for thepurpose of error correction. During this write-to-read transition,residual magnetization may remain as a function of the direction thatflux last passed through the head, that is, the direction of the fluxused to write the last piece of data.

As a result of this residual magnetization, the base line of the pulsesderived from one burst is shifted, but a similar shift is not present inthe pulses derived from the other burst. Because of the offset in thebursts, the residual magnetism at one end of the gap will have theeffect of biasing the base line of the pulses derived from the burstthat is offset in the direction of that end. A similar bias effect isnot present in the pulses derived from the burst that is offset in thedirection of the other end of the gap. Consequently, when the pulses arefull-wave rectified, prior to detecting their peaks, as is conventional,the peaks detected from the pulses that have been biased above (orbelow) the base line will be greater than the peaks which are detectedfrom the pulses that have not been so biased. Thus, even though the headmay be centered accurately on a track, the peak voltage level detectedfrom the pulses derived from one burst will differ from the peak voltagethat is detected from the pulses derived from the other burst. This peakdifferential is interpreted erroneously as a tracking error. Hence, eventhough the head is in proper registration with a track, a "correction"will be made, with the result that the head now will be shifted intomisregistration. This occurrence, which typically follows awrite-to-read transition, is referred to as Write Induced PositionError, or WIPE.

Another difficulty encountered in typical disk drive operations is thegeneral inability of a typical pulse detecting channel to discriminatebetween offset errors and gain errors. An offset error is produced whenthe preamplifier circuit normally included in the write channel, havingbeen heated by a write current when writing data, generates an outputoffset different from that from the immediately preceding readoperation. This new offset then is coupled into the read channel by ahigh pass filter. As a result, the pulses produced by the read channelexhibit a corresponding shift either upwardly or downwardly, dependingupon the direction in which the offset is generated.

At a write-to-read transition, as when the servo pattern is sensedfollowing a write operation, the gain of the playback amplifiers, andmore particularly, the gain of the read channel, may need adjustment.For example, at the beginning of the servo pattern, the gain of the readchannel may be too high and automatic gain control (AGC) operates toadjust this gain to its proper level. Conversely, if the gain of theread channel is too low at the beginning of the servo pattern, AGCoperation increases the gain to its proper level.

The envelope of the pulses derived from, for example, the offset servobursts, is used to detect both offset and gain errors. However, when thepulses are full-wave rectified, it often is difficult, if notimpossible, to discriminate between gain and offset errors.Consequently, the proper corrective operation is not easily implemented.

OBJECTS OF THE INVENTION

Therefore, it is an object of the present invention to provide animproved pulse peak detector which overcomes the aforenoteddisadvantages of prior art peak detectors which use full-waverectification.

Another object of this invention is to provide an improved discriminatorwhich readily discriminates between offset and gain errors in a readchannel.

A further object of this invention is to provide an improved pulse peakdetector which is not influenced by residual magnetism in the read/writehead.

An additional object of the present invention is to provide an improvedpulse detector for use in AGC level detection as well as for servoposition detection.

Various other objects, advantages and features of the present inventionwill become readily apparent from the ensuing detailed description, andthe novel features will be particularly pointed out in the appendedclaims.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention, apparatus is providedfor peak detecting pulses recovered from a magnetic disk by a magnetichead which may exhibit residual magnetism. Differential pulses recoveredby the head are supplied as first and second pulse trains to first andsecond peak detectors, respectively. The peaks detected in the firsttrain of differential pulses are combined with the peaks detected in thesecond train; and an output signal indicative of the pulse peak isproduced as a function of the combined peaks.

As a feature of this aspect, a combining circuit is used to average thepeaks detected in both trains of differential pulses. The combiningcircuit preferably comprises a summing circuit.

As another feature of this aspect, each peak detector is comprised of adifferential amplifier supplied with both trains of differential pulses.A capacitor stores an output voltage produced by the differentialamplifier, whereby the capacitor of one peak detector stores peaks of,for example, noninverted pulses and the capacitor of the other peakdetector stores peaks of inverted pulses. Advantageously, a differentialhead produces the two trains of differential pulses as inverted andnoninverted pulses, respectively.

As yet a further feature of this aspect, each peak detector includes areset circuit for resetting the capacitor therein. The capacitor whichstores peaks of the noninverted pulses is reset in response to the nextnon-inverted pulse before being charged to the level of thisnext-following non-inverted pulse and, similarly, the capacitor whichstores peaks of the inverted pulses is reset in response to the nextinverted pulse before being charged to the level of this next-followinginverted pulse. This is achieved by operating one reset switch inresponse to an inverted pulse and operating another reset switch inresponse to a noninverted pulse.

As another aspect of this invention, apparatus is provided fordiscriminating between offset errors and gain errors in a pulsedetecting, or read, channel of a disk drive. First and second peakdetectors, preferably of the aforementioned type, detect positive andnegative peaks, respectively, of pulses that are detected from the diskdrive. A comparison circuit compares the positive and negative peaks toa set of reference levels, and a gain error is indicated if the positiveand negative peaks exhibit substantially similar relationships withrespect to the reference levels, whereas an offset error is indicated ifthe positive and negative peaks exhibit dissimilar relationships withrespect to the reference levels.

As a feature of this aspect, the reference levels comprise a relativelyhigh reference level and a relatively low reference level. Thecomparator circuit compares the positive peaks to the high and lowreference levels, respectively, and the negative peaks also are comparedto the respective high and low reference levels. A gain error isindicated when the positive and negative peaks both exceed the highreference level or both are less than the low reference level; and anoffset error is indicated when the positive (or negative) peaks exceedthe high reference level and the negative (or positive) peaks are lessthan the low reference level.

As another feature of this aspect, the gain error is indicated by afirst coincidence circuit which senses when the positive peaks and thenegative peaks both exceed the high reference level and a secondcoincidence circuit which senses when the positive peaks and thenegative peaks both are less than the low reference level. An offseterror is indicated by a further coincidence circuit which senses whenthe positive peaks exceed one reference level and the negative peaksexceed the other.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the present invention solely thereto, will best beappreciated in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a preferred embodiment of a peakdetector in accordance with the present invention;

FIGS. 2A-2H are waveform diagrams which are useful in understanding theoperation of the peak detector shown in FIG. 1;

FIGS. 3A-3H are waveform diagrams which also are helpful inunderstanding the operation of the peak detector shown in FIG. 1;

FIGS. 4A-4C are waveform diagrams which are useful in understanding themanner in which the gain/offset error discriminator of the presentinvention operates to detect an offset error;

FIGS. 5A-5D are waveform diagrams which are helpful in understanding themanner in which the gain/offset error discriminator of the presentinvention operates to detect a gain error; and

FIG. 6 is a schematic diagram of a preferred embodiment of thegain/offset error discriminator of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is illustrated a preferred embodiment ofpeak detecting apparatus in accordance with the present invention. Thepeak detecting apparatus is comprised of peak detectors 10 and 12, acombining circuit 14 and an output circuit 16. The peak detectors aresupplied with differential pulses produced by a differential playbacktransducer, or head 100. A differential head normally is used to read orwrite information on a magnetic disk, such as a hard disk, and isadapted to produce two trains of pulses, one of which is an invertedversion of the other. Typical waveforms of these differential pulses,including their respective relationships relative to each other also areillustrated in FIG. 1. It will be recognized that the positive andnegative pulses in both trains are positive-going and negative-goingwith respect to a base line which may be considered an AC zero level.For convenience, the pulses supplied by head 100 to peak detector 10 arereferred to as noninverted or positive pulses and the pulses supplied topeak detector 12 are referred to as inverted or negative pulses.

Peak detector 10 is comprised of a differential amplifier 101, apeak-hold capacitor 103 and a coupling transistor 105. The differentialamplifier includes noninverting and inverting inputs across which thedifferential pulses are supplied. As illustrated, the noninverting inputof differential amplifier 101 is coupled to head 100 to receive thenoninverted pulses, and the inverting input of the differentialamplifier is coupled to the head to receive the inverted pulses.

Transistor 105 is illustrated as an npn transistor whose base-emittercircuit couples the output of differential amplifier 101 to peak-holdcapacitor 103. In this configuration, it will be appreciated that peakdetector 10 functions to detect the positive peaks of the noninvertedpulses supplied thereto by head 100.

Peak detector 12 is comprised of a differential amplifier 102, apeak-hold capacitor 104 and a transistor 106 for coupling the capacitorto the differential amplifier. It is seen that peak detector 12 is ofsubstantially the same configuration as peak detector 10 and functionsto detect the positive peaks of the inverted pulses supplied thereto byhead 100.

Peak detector 10 also includes a reset circuit for resetting capacitor103 before the detection of the next positive pulse in the noninvertingpulse train. This reset circuit includes a transistor 107, whosecollector-emitter circuit is connected in parallel across capacitor 103and in series with a current source transistor 110, and a diode 145poled to produce a forward bias voltage drop V_(be) from the output ofamplifier 101 to the base electrode of transistor 107.

A similar reset circuit is coupled to capacitor 104 in peak detector 12.Here, transistor 108 has its collector-emitter circuit connected inparallel with capacitor 104 and in series with current source 110. Adiode 146 is poled to produce a forward bias voltage drop V_(be) fromthe output of amplifier 102 to the base of transistor 108.

The reset circuits coupled to peak detectors 10 and 12 also include adelay capacitor 112 which is connected in parallel with current source110 and, thus, is coupled to the respective emitters of transistors 107and 108. It will be seen that the voltage across delay capacitor 112establishes the emitter voltage of transistors 107 and 108, and as thisvoltage decays, the threshold level by which the transistors turn on todischarge capacitors 103 and 104 decreases. In one embodiment, delaycapacitor 112 functions to reset capacitors 103 and 104 after a 1microsecond delay in the absence of pulses supplied by head 100.

Before describing the remaining circuitry included in the peak detectorapparatus, a brief description of the operation of the reset circuit isprovided. Capacitor 112 is charged alternately through transistor 107and then through transistor 108 in response to positive pulses producedby amplifier 101 and amplifier 102, respectively. In between thesepulses, capacitor 112 discharges through current source 110. It isappreciated that when capacitor 103 stores a pulse peak voltage, theemitter of transistor 105 is back-biased and, thus, is not renderedconductive, or turned on, by a noninverted pulse supplied to its base byamplifier 101. However, transistor 107 is not back-biased (becausecapacitor 112 will have discharged through current source 110) and,therefore, it is turned on to discharge capacitor 103 therethrough tocapacitor 112. Accordingly, capacitor 103 discharges rapidly to reducethe voltage at the emitter of transistor 105. When this voltage fallsbelow the base voltage thereof, transistor 105 is turned on to chargecapacitor 103 to the peak voltage of the pulse now produced at theoutput of amplifier 101. Hence, capacitor 103 is reset in response tothe next-following non-inverted pulse before it is charged to the peaklevel of that non-inverted pulse. A similar reset operation is carriedout to reset capacitor 104 in response to the next-following invertedpulse produced at the output of amplifier 102.

Combining circuit 14 is comprised of summing resistors 141 and 142connected to capacitors 103 and 104 by buffers 22 and 24, respectively.These summing resistors function to sum the peak-held voltages acrosscapacitors 103 and 104; and in the preferred embodiment, these resistorsare of equal resistance values so as to average the peak-held voltages.

Output circuit 16 is adapted to sample the averaged peak-held voltagesand to supply those samples to a microprocessor. If these samples arederived from servo pulses, the samples are used as tracking errorsignals for carrying out a servo-controlled error correcting operation.Tracking error signals normally take the form of "normal" signals N and"quadrature" signals Q. In implementing a tracking control operation,the microprocessor typically is provided with N, N, Q and Q signals.Sampling circuit 16 operates to sample the averaged peak-held voltagesso as to produce samples corresponding to these N, N, Q and Q signals.Accordingly, the output circuit includes sampling switches 161a, 161b,161c and 161d coupled to combining circuit 14 and operable at differentsampling phases in response to sampling pulses supplied thereto by asample pulse generator 164. The phases of these sample pulses correspondto the N, N, Q and O signals.

The samples produced by the respective sampling switches are stored oncapacitors 162a, 162b, 162c and 162d, respectively. These capacitorsoperate as hold capacitors; and it is seen that the combination ofsampling switches 161 with hold capacitors 162 comprise sample-and-holdcircuits. In addition, and advantageously, the combination of holdcapacitors 162 and summing resistors 141, 142 functions as a low passfilter to band limit noise. Read-out switches 163a, 163b, 163c and 163dare coupled to hold capacitors 162a, 162b, 162c and 162d, respectively,and these read-out switches operate in timed relationship in response tosample pulses supplied thereto by the sample pulse generator. Theseread-out switches operate one-at-a-time to supply a serial stream ofpeak-held samples N, N, Q and Q which are coupled to an output terminal166 by a buffer amplifier 165, from which the samples are supplied tothe microprocessor (not shown).

The manner in which the peak detector circuitry shown in FIG. 1 operatesto produce an accurate representation of pulse peaks, notwithstandingthe presence of residual magnetism in head 100, now will be described inconjunction with the waveform diagrams of FIGS. 2A-2H. For convenience,this operation is described with reference to reproducing servo pulses;and these waveform diagrams result from an accurate registration of head100 relative to the track in which the servo pattern is recorded. FIG.2A illustrates this servo pattern as a burst of magnetic domains Acomprising normal signals N followed by a burst of magnetic domains Bcomprising normal signals N, these bursts being offset relative to thecenter line of the track, first to one side and then to the other. Atracking error is detected by comparing the peaks of the pulses producedin response to scanning burst A to the peaks produced in response toscanning burst B. A similar pattern of quadrature bursts also may beprovided; but for ease of understanding the present invention, furtherdescription of these quadrature bursts is not needed and not provided.Ideally, when head 100 is in proper registration with the illustratedtrack, these two sets of peaks, that is, the peaks produced when burst Ais scanned and the peaks produced when burst B is scanned, are equal.

FIG. 2B illustrates the pulses produced when bursts A and B are scannedby a read/write head having residual magnetism. It is assumed that thisresidual magnetism is such that the base line of the pulses producedwhen burst A is scanned is effectively shifted in the downward directionrelative to the base line when burst B is scanned. Thus, although thepositive and negative excursions of the pulses produced when burst A isscanned are equal to each other and are also equal to the positive andnegative excursions of the pulses produced when burst B is scanned, itis seen that the positive peaks of the burst A pulses are less than thepositive peaks of the burst B pulses.

In the prior art, the pulses shown in FIG. 2B are full-wave rectified,resulting in the positive-going pulses of FIG. 2C. It is seen that whenthe negative-going pulses derived from burst A are rectified, the peaksthereof exceed the peaks of the positive-going pulses in both the burstA pulses and the burst B pulses. Hence, when the pulses shown in FIG. 2Care peak detected, the peak-held level derived from the burst A pulsesis greater than the peak-held level derived from the burst B pulses.This differential in the detected peaks is interpreted as a trackingerror; and even though head 100 is in proper registration with the track(as shown in FIG. 2A), the tracking control servo tends to shift theposition of the head to make the detected peaks derived from burst Aequal to the detected peaks derived from burst B. As a result, head 100is shifted from proper registration into misregistration.

FIG. 2D illustrates the noninverted train of differential pulsesproduced by head 100 and FIG. 2E illustrates the inverted train of thedifferential pulses. The pulses illustrated in FIG. 2D are the same asthose illustrated in FIG. 2B. Thus, the base line of the inverted pulsesderived from burst A is shifted downward relative to the base line ofthe noninverted pulses derived from burst B. In FIG. 2E, it is seen thatthe base line of the inverted pulses derived from burst A is shiftedupward relative to the base line of the inverted pulses derived fromburst B.

FIG. 2F illustrates the peak-held voltage to which capacitor 103 ischarged in response to the positive-going peaks of the noninvertedpulses detected by peak detector 10. It is seen that the peak-heldvoltage derived from the burst A pulses is less than the peak-heldvoltage derived from the burst B pulses. The ripples in the peak-heldvoltage levels are caused by the gradual discharge of capacitor 103 andthe resetting thereof prior to the detection of the next positive-goingpulse peak. Similarly, FIG. 2G illustrates the peak-held voltageproduced by capacitor 104 representing the peaks of the positive-goingpulses of the inverted pulse train, as detected by peak detector 12. Thepeak-held voltage waveform shown in FIG. 2G is produced in response tothe pulse train illustrated in FIG. 2E. It is seen that the peak-heldvoltage derived from burst A is greater than the peak-held voltagederived from burst B, as caused by the shift in the base line shown inFIG. 2E because of the residual magnetism in head 100. A comparison ofFIGS. 2F and 2G illustrates that the peak-held voltages derived fromburst B are equal, but the peak-held voltages derived from burst Adiffer from each other. This difference is, of course, attributed to theshift in the base line, as shown in FIGS. 2D and 2E.

FIG. 2H illustrates the summed peak-held voltages produced by combiningcircuit 14. In the preferred embodiment, summing resistors 141 and 142average the peak-held voltages. It is apparent that the summed oraveraged peak-held voltages derived from burst A is equal to the summedor averaged peak-held voltages derived from burst B. Thus, even thoughthe peak-held voltages produced by, for example, peak detector 10 inresponse to the burst A pulses differ from the peak-held voltagesderived by this peak detector from the burst B pulses, the summed oraveraged peak-held voltages do not exhibit this differential. Samples ofthe summed or averaged peak-held voltages produced by output circuit 16are supplied to the microprocessor, and these samples are of equalmagnitude, notwithstanding the residual magnetism in head 100. Thus,even though residual magnetism may be present, if head 100 is in properregistration with the track being scanned thereby, a tracking errorindication is not falsely produced.

Let it now be assumed that head 100 is misaligned with respect to atrack. FIG. 3A illustrates this misregistration of the head; and it isseen that the head is shifted in the downward direction. For theapplication of the present invention to recover servo pulses, the servopulses recovered from burst A are of a smaller magnitude than the pulsesrecovered from burst B. Assuming once again the presence of residualmagnetism in head 100, FIG. 3B illustrates the waveform of the pulsesrecovered from burst A and those recovered from burst B. FIG. 3B furtherillustrates the base line of the pulses recovered from burst A isshifted downwardly (i.e. in the negative direction) as is caused by theaforementioned residual magnetism.

FIG. 3C illustrates the waveform of the pulses recovered from bursts Aand B following full wave rectification thereof, as is used by prior arttracking error detectors. It will be seen that the negative-going burstA pulses of FIG. 3B, when rectified, exhibit a greater magnitude thanthe positive-going burst A pulses, and this is attributed to thedownward shifting in the base line of the pulses. In comparison, FIG. 3Dillustrates the noninverted pulse train of the differential pulsesgenerated by head 100 and supplied to peak detector 10, and FIG. 3Eillustrates the inverted pulse train of the differential pulses suppliedby the head to peak detector 12. A comparison of FIG. 3D toaforedescribed FIG. 2D and, likewise, a comparison of FIG. 3E toaforedescribed FIG. 2E, illustrates the results of the misregistrationof head 100 relative to the track being scanned thereby. The amplitudesof the noninverted pulse train recovered from burst A are much less thanthe amplitudes of the noninverted pulse train recovered from burst B.Likewise, the amplitudes of the inverted pulse train recovered fromburst A are less than the amplitudes of the inverted pulse trainrecovered from burst B. FIGS. 3D and 3E also illustrate the voltage towhich capacitors 103 and 104 are charged, respectively, in response tothe positive peaks detected by peak detectors 10 and 12 from therespective pulse trains supplied thereto by head 100.

The peak-held voltage to which capacitor 103 is charged is illustratedin FIG. 3F; and the peak-held voltage to which capacitor 104 is chargedis illustrated in FIG. 3G. The difference between the peaks of thepulses recovered from burst A and the peaks of the pulses recovered fromburst B is readily apparent, even when head 100 exhibits residualmagnetism. The peak-held voltages are combined by summing resistors 141and 142, resulting in the summed, or averaged, peak-held voltageillustrated in FIG. 3H. The effect of the residual magnetism iscanceled; and FIG. 3H clearly illustrates the peak-held voltage derivedfrom burst B exceeds the peak-held voltage derived from burst A, therebyindicating a tracking error. The combined peak-held voltages produced bycombining circuit 14 are sampled and supplied to other circuitry, suchas a microprocessor, for further processing or utilization.

The description set forth hereinabove with respect to FIG. 1 is directedto the peak detecting apparatus of the present invention, whereby pulselevels are detected accurately notwithstanding residual magnetism whichmay be present in the read head. The following discussion is directed tooffset/gain error detector 120 which is supplied with the peak-heldvoltages stored on capacitors 103 and 104 and which operates todiscriminate between an offset error and a gain error in the servo pulsedetecting channel.

Before describing the structure and operation of the offset/gain errordetector, reference is made to FIGS. 4A and 5A which show the pulsesthat are produced when pulse information, such as a servo burst pattern,is scanned. FIG. 4A illustrates a typical waveform recovered from aservo burst when the read head, such as head 100, initially exhibits anoffset. As mentioned previously, an offset may be produced as a resultof heat generated during a preceding write operation. FIG. 4A may bethought of as being illustrative of the noninverted pulse train producedby head 100 and supplied to peak detector 10 with an offset. As the ACcouple recovers from the applied offset, the positive peak envelope andthe negative peak envelope of the pulses shift as illustrated.

The waveform of FIG. 4A is to be contrasted with that of FIG. 5A whichillustrates pulses that are recovered by the pulse detecting channelwhose gain initially is too high and is adjusted while the pulses arerecovered. As the gain of the pulse detecting channel is reduced, thepositive peak envelope and negative peak envelope likewise are reduced.

Since the typical pulse detecting channel relies upon full waverectification of the pulses, FIGS. 4B and 5B illustrate the full waverectified pulses produced from the waveforms shown in FIGS. 4A and 5A,respectively. Notwithstanding the different shapes of the envelopes ofthe pulses due to offset error (shown in FIG. 4A) and gain error (shownin FIG. 5A), the envelopes recovered from full wave rectification aresubstantially similar. Thus, envelope V_(PP) shown in FIG. 4B, producedby peak detecting the full wave rectified pulses resulting from theoffset error shown in FIG. 4A, is substantially similar to the envelopeV'_(PP) shown in FIG. 5B, produced by peak detecting the full rectifiedpulses resulting from the gain error shown in FIG. 5A. Consequently,since envelopes V_(PP) and V'_(PP) are substantially the same, it isdifficult, if not impossible, to discriminate between an offset errorand a gain error.

FIG. 4C illustrates the positive peak and negative peak envelopesderived from the peak-held voltages on capacitors 103 and 104. Inparticular, envelope A is derived from the detected peaks of thenoninverted pulse train produced by head 100 and envelope B is derivedfrom the detected peaks of the inverted pulse train produced by thehead. Similarly, in FIG. 5C, envelope A' is derived from the detectedpeaks of the noninverted pulse train of the differential pulses producedby head 100 and envelope B' is derived from the detected peaks of theinverted pulse train and is seen to be substantially the same asenvelope A'. A comparison of FIGS. 4C and 5C indicates that, by usingthe envelopes recovered from the detected peaks of both the noninvertedand inverted pulse trains, discrimination between an offset error and again error is readily achieved.

As will be described below, this discrimination is carried out inaccordance with the following observation: the waveform of envelope Aand the waveform of envelope B are inversely related to each other, thatis, they exhibit dissimilar relationships, when an offset error ispresent, as shown in FIG. 4C, but are substantially identical to eachother (A'=B'), that is, they exhibit substantially similarrelationships, when a gain error is present, as shown in FIG. 5C. Thepresent invention proceeds by detecting this dissimilar or similarrelationship.

A set of reference levels comprised of relatively high and low thresholdlevels V_(H) and V_(L) are selected, as shown in broken lines in FIGS.4C and 5C. These threshold levels are such that, in the absence of anerror, they will not be crossed or traversed by envelopes A and B (FIG.4C) or by envelopes A' and B' (FIG. 5C). Consequently, if the magnitudeof envelope A derived from the positive peaks of the pulses is greaterthan the higher threshold level V_(H) while, at the same time, theenvelope B derived from the negative peaks of the pulses is less thanthe lower threshold level V_(L), it is concluded that an error in thepulse detecting channel is present and this error is an offset error.However, if the envelope A' derived from the positive peaks of thepulses as well as the envelope B' derived from the negative peaks of thepulses both exceed the higher threshold level V_(H), as shown in FIG.5C, or both are less than the lower threshold level V_(L), as shown inFIG. 5D, it is concluded that an error in the servo pulse detectingchannel is present and this error is a gain error. If the envelopes Aand B (or A' and B') are disposed between the threshold levels V_(H) andV_(L), it is concluded that no error is present.

FIG. 5D illustrates the envelopes A' and B' derived from the positiveand negative pulse peaks, respectively, when a gain error is of the typethat requires an increase in the gain of the pulse detecting channel.This is in comparison to FIG. 5C which illustrates the envelopes derivedfrom the positive and negative pulse peaks when the gain error in thepulse detecting channel requires a reduction. As the gain increases tocorrect the gain error, the amplitudes of the pulses likewise increase,as illustrated in FIG. 5D. For the error condition wherein the gain ofthe read channel must increase, it is seen that both envelopes initiallyare less than the lower threshold level V_(L).

The manner in which the envelopes of the positive and negative peaks ofthe pulses are derived and detected for the purpose of discriminatingbetween offset and gain error now is described with reference to FIG. 6,which is a schematic diagram of offset/gain error discriminator 120,shown in FIG. 1. This discriminator is comprised of a comparison circuitfor comparing the positive peaks of the pulses to the aforementioned setof reference levels, another comparison circuit for comparing thenegative peaks of the pulses to these reference levels and a gate arrayfor processing the results of such comparison. More particularly, thepeak-held voltage stored on capacitor 103 is proportional to thepositive peaks of the noninverted pulses produced by head 100, and thispeak-held voltage is coupled in common to comparators 124 and 125 by wayof a buffer 121. Similarly, the peak-held voltage stored on capacitor104 is proportional to the positive peaks of the inverted pulsesproduced by head 100, and this is related to the negative peaks of thenoninverted pulses as shown in, for example, FIG. 2E. In accordance withthe desired operation of this aspect of the present invention, thepeak-held voltage corresponding to the negative peaks of the pulses,that is, the peak-held voltage stored on capacitor 104, is coupled by abuffer 122 to comparators 126 and 127.

Comparator 124 is adapted to sense when the positive peaks of the pulsesexceed the higher threshold voltage V_(H), and includes a noninvertinginput coupled to receive the positive peaks and an inverting inputcoupled to receive the higher threshold voltage V_(H). Comparator 125 isadapted to sense when the positive peaks of the pulses are less than thelower threshold voltage V_(L) and includes a noninverting input coupledto receive the lower threshold level V_(L) and an inverting inputcoupled to receive the positive peaks of the pulses.

Comparator 126 is adapted to sense when the negative peaks of the pulsesare less than the lower threshold voltage V_(L) and includes anoninverting input coupled to receive the lower threshold voltage V_(L)and an inverting input coupled to buffer 122 to receive the negativepeaks of the pulses. Comparator 127 is adapted to sense when thenegative peaks of the pulses exceed the higher threshold voltage V_(H)and includes a noninverting input coupled to buffer 122 to receive thenegative peaks of the pulses and an inverting input coupled to receivethe higher threshold voltage V_(H).

The gate array is comprised of AND gates 128-131 and 0R gates 132 and133. AND gate 128 is adapted to detect when the positive peaks of thepulses exceed the higher threshold level V_(H) and the negative peaks ofthe pulses are less than the lower threshold level V_(L). Accordingly,AND gate 128 is coupled to the outputs of comparators 124 and 126.

It is appreciated, from FIG. 1, that since differential pulses aresupplied to peak detectors 10 and 12, it is just as likely for peakdetector 12 to be supplied with the noninverted pulse train as it is forpeak detector 10 to be supplied with this noninverted pulse train.Consequently, the peak-held voltage stored in capacitor 103 may be lessthan the lower threshold level V_(L) at the same time that the peak-heldvoltage stored on capacitor 104 exceeds the higher threshold voltageV_(H). Accordingly, AND gate 129 is coupled to the outputs ofcomparators 125 and 127 to sense this condition, that is, to detect whenthe peak-held voltage stored on capacitor 103 is less than the lowerthreshold level V_(L) and the inverted peak-held voltage stored oncapacitor 104 exceeds the higher threshold level V_(H). OR gate 132 iscoupled to AND gates 128 and 129 to produce an offset error indicationwhen either AND gate is supplied with coinciding signals.

AND gate 130 is coupled to the outputs of comparators 125 and 126 and isadapted to sense when the positive peaks as well as the negative peaksof the pulses both are less than the lower threshold level V_(L). ANDgate 131 is coupled to comparators 124 and 127 and is adapted to sensewhen the positive peaks and the negative peaks of the pulses both exceedthe higher threshold level V_(H). OR gate 133 is coupled to AND gates130 and 131 and is adapted to produce a gain error indication wheneither of these AND gates is supplied with coinciding signals.

The manner in which offset/gain error discriminator 120 operates nowwill be discussed in conjunction with the waveforms shown in FIGS. 4 and5. Let it be assumed that an offset error is present and that one of thetrains of pulses produced by head 100, such as, for example, servopulses, appears as shown in FIG. 4A. From the foregoing description, itis recalled that capacitor 103 stores a peak-held voltage derived fromthe positive-going peaks of the noninverted pulse train and capacitor104 stores the peak-held voltage derived from the positive-going peaksof the inverted pulse train. Under ideal conditions, the peak-heldvoltage stored on capacitor 103 is substantially equal to the peak-heldvoltage stored on capacitor 104.

The present example has assumed the presence of an offset error and,therefore, the peak-held voltage supplied to comparators 124 and 125corresponds to envelope A, shown in FIG. 4C, and the peak-held voltagesupplied to comparators 126 and 127 is illustrated as envelope B. Theseenvelopes exhibit dissimilar relationships with respect to the higherand lower threshold levels. Comparator 124 produces a positive outputsignal when envelope A, supplied by capacitor 103, exceeds thresholdlevel V_(H). Likewise, comparator 126 produces a positive signal whenenvelope B, supplied from capacitor 104, is less than the lowerthreshold level V_(L). AND gate 128 senses the coincidence of thepositive output signals produced by comparators 124 and 126; and anoffset error indication is produced by OR gate 132.

When envelope A exceeds the high threshold level V_(H), comparator 125produces a low or negative output signal which inhibits AND gate 130.Likewise, when envelope B is less than the lower threshold level V_(L),comparator 127 produces a low or negative output signal to inhibit ANDgate 131. Consequently, a gain error indication is not provided.

Now, let it be assumed that a gain error is present, resulting in pulsetrains having the waveform shown in FIG. 5A. Assuming that this gainerror begins as a high gain and is corrected by reducing its level, thepeak-held voltage stored on capacitor 103 may appear as indicated byenvelope A' in FIG. 5C and the peak-held voltage stored on capacitor 104may appear as the envelope B' in FIG. 5C. Envelope B' is substantiallysimilar to envelope A', as shown in FIG. 5C Comparator 124 produces apositive output signal when envelope A' exceeds the higher thresholdlevel V_(H) and comparator 127 likewise produces a positive outputsignal when inverted envelope B' exceeds the higher threshold level. ANDgate 131 senses the coinciding positive output signals produced bycomparators 124 and 127; and OR gate 133 produces a gain errorindication.

Alternatively, let it be assumed that the gain error in the pulsedetecting channel is present as a low gain, which thereafter iscorrected by increasing its level. With this condition, the peak-heldvoltage stored on capacitor 103 appears as envelope A' in FIG. 5D, andthe peak-held voltage stored on capacitor 104 appears as the envelopeB', also shown in FIG. 5D. Now, envelope B' and envelope A' are seen toexhibit substantially similar relationships with respect to thethreshold levels. Comparator 125 produces a positive output signal whenenvelope A' is less than the lower threshold voltage V_(L), andcomparator 126 likewise produces a positive output signal when envelopeB' is less than the lower threshold level. AND gate 130 senses thecoinciding positive output signals produced by comparators 125 and 126;and OR gate 133 produces a gain error indication.

Thus, it is seen that, when the positive and negative peaks derived fromthe reproduced pulses exhibit a substantially similar relationship withrespect to the set of reference levels V_(H) and V_(L), a gain error isindicated. However, if the positive and negative peaks exhibitdissimilar relationships with respect to the reference levels V_(H) andV_(L), an offset error is indicated. The ambiguity associated with priorart error discriminators thus is avoided.

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the appended claims be interpreted ascovering the embodiments described herein and all equivalents thereto.

What is claimed is:
 1. Apparatus for peak detecting pulses recoveredfrom a magnetic disk by a magnetic head, said apparatus comprising:inputmeans for concurrently supplying non-full-wave rectified first andsecond trains of differential pulses recovered from the same track bysaid magnetic head, said first non-full-wave rectified train ofdifferential pulses exhibiting a baseline shift relative to said secondnon-full-wave rectified train of differential pulses as a result ofasymmetric residual magnetism of said magnetic head; first and secondpeak detecting means connected to said input means for detecting peaksof said first and second non-full-wave rectified trains ofbaseline-shifted differential pulses, respectively; adding means foradding the peaks detected in said first non-full-wave rectified train ofdifferential pulses with the peaks detected in said second non-full-waverectified train of differential pulses; and output means for providingan output signal indicative of pulse peaks as a function of the addedpeaks.
 2. The apparatus of claim 1 wherein said adding means comprisesaveraging means for averaging the peaks detected in said first train ofdifferential pulses and the peaks detected in said second train ofdifferential pulses.
 3. The apparatus of claim 1 wherein said addingmeans comprises summing means for summing the peaks detected in saidfirst train of differential pulses with the peaks detected in saidsecond train of differential pulses.
 4. The apparatus of claim 1 whereinsaid magnetic head comprises a differential head for concurrentlyproducing first and second trains of noninverted and inverted pulses,respectively, the noninverted and inverted pulses exhibiting a baselineshift relative to each other; and said input means comprises means forreceiving the noninverted and inverted pulses as the respective firstand second trains of differential pulses.
 5. The apparatus of claim 1wherein said output means comprises sampling means for sampling theadded peaks to produce said output signal.
 6. The apparatus of claim 1,wherein said input means concurrently supplies third and fourthnon-full-wave rectified trains of non-baseline-shifted differentialpulses sequentially after said concurrently supplied first and secondnon-full-wave rectified trains of baseline-shifted differential pulses,said first, second, third and fourth trains of differential pulses beingrecovered by the same magnetic head from respective bursts of magneticdomain recorded in the same track on said magnetic disk, the respectivebursts being offset relative to a center line of the track.
 7. Theapparatus of claim 6, wherein said magnetic head comprises adifferential head for concurrently producing first and second trains ofnoninverted and inverted pulses, respectively, and for concurrentlyproducing third and fourth trains of noninverted and inverted pulses,respectively, the first and second trains of noninverted and invertedpulses exhibiting a baseline shift to each other, and the third andfourth trains of noninverted and inverted pulses not exhibiting abaseline shift to each other; and said input means comprises means forreceiving the first and second trains of noninverted and inverted pulsesas the respective first and second trains of differential pulses and forreceiving the third and fourth trains of noninverted and inverted pulsesas the respective third and fourth trains of differential pulses. 8.Apparatus for peak detecting pulses recovered from a magnetic disk by amagnetic head said apparatus comprising:input means for supplyingnon-full-wave rectified first and second trains of noninverted andinverted differential pulses, respectively, recovered by said magnetichead comprising a differential head for concurrently producing saidnoninverted and inverted pulses, said noninverted and inverted pulsesexhibiting a baseline shift relative to each other as a result ofasymmetric residual magnetism of said magnetic head; first and secondpeak detecting means connected to said input means for detecting peaksof said first and second non-full-wave rectified trains ofbaseline-shifted noninverted and inverted differential pulses,respectively, each of said first and second peak detecting meanscomprising differential amplifier means supplied with said noninvertedand inverted pulses, capacitor means for storing an output voltageproduced by said differential amplifier means, and transistor means forcoupling said capacitor means to said differential amplifier means tostore peak levels of said output voltage such that the capacitor meansof said first peak detecting means stores peaks of said noninvertedpulses and the capacitor means of said second peak detecting meansstores peaks of said inverted pulses; adding means for adding the peaksdetected in said first non-full-wave rectified train of noninvertedpulses with the peaks detected in said second non-full-wave rectifiedtrain of inverted pulses; and output means for providing an outputsignal indicative of pulse peaks as a function of the added peaks. 9.The apparatus of claim 8 wherein said adding means comprises resistivesumming means coupled to each of said capacitor means for averaging thestored peaks of the noninverted and inverted pulses and filtering noise.10. The apparatus of claim 8 wherein each of said first and second peakdetecting means further comprises reset means for resetting thecapacitor means of said first peak detecting means in response to anext-following non-inverted pulse and for resetting the capacitor meansof said second peak detecting means in response to a next-followinginverted pulse.
 11. The apparatus of claim 10 wherein said reset meanscomprises first reset switch means coupled to the capacitor means ofsaid first peak detecting means and activated in response to thenext-following non-inverted pulse, and second reset switch means coupledto the capacitor means of said second peak detecting means and activatedin response to the next-following inverted pulse.
 12. The apparatus ofclaim 11 wherein said reset means further comprises delay means coupledto said first and second reset switch means for enabling said first andsecond reset switch means a predetermined delay time after peak levelsare stored in the absence of pulses produced by said differential head.13. The apparatus of claim 12 wherein said first and second reset switchmeans comprise first and second reset transistor means, respectively,and said delay means comprises a delay capacitor, said first resettransistor means having a collector-emitter circuit for coupling thecapacitor means of said first peak detecting means to said delaycapacitor and said second reset transistor means having acollector-emitter circuit for coupling the capacitor means of saidsecond peak detecting means to said delay capacitor
 14. The apparatus ofclaim 13 wherein said reset means additionally comprises a currentsource connected in series with the collector-emitter circuits of saidfirst and second reset transistor means.
 15. Apparatus for peakdetecting pulses recovered from a magnetic disk by a magnetic head, saidapparatus comprising:input means for sequentially supplying first andsecond pairs of non-full-wave rectified trains of differential pulsesrecovered by said magnetic head from first and second trains ofdifferential pulses, respectively, recorded in a track and physicallyoffset from one another on said track, each of said first and secondpairs of non-full-wave rectified trains of differential pulses having afirst train and a second train of differential pulses, said first trainof said first pair exhibiting a baseline shift relative to the secondtrain of said first pair as a result of asymmetric residual magnetism ofsaid magnetic head; first and second peak detecting means connected tosaid input means for detecting peaks of said first and second trains,respectively, of one of said first and second pairs of non-full-waverectified trains and supplying a first detected peak and a seconddetected peak as respective outputs; adding means for adding said firstdetected peak with said second detected peak; and output means forproviding an output signal indicative of pulse peaks as a function ofthe added peaks.
 16. The apparatus of claim 15, wherein said addingmeans comprises averaging means for averaging the first detected peakand the second detected peak.
 17. The apparatus of claim 15, whereinsaid adding means comprises summing means for summing the first detectedpeak with the second detected peak.
 18. The apparatus of claim 15,wherein said magnetic head comprises a differential head forconcurrently producing first and second trains of noninverted andinverted pulses, respectively; and said input means comprises means forreceiving the noninverted and inverted pulses as the respective firstand second trains of one of said first and second pairs of non-full-waverectified trains.
 19. The apparatus of claim 15, wherein said inputmeans concurrently supplies said first and second trains of said firstpair of non-full-wave rectified trains and concurrently supplies saidfirst and second trains of said second pair of non-full-wave rectifiedtrains.